1. Field of the Invention
The present invention relates to an oxide ceramic fiber/oxide ceramic composite material which is an oxide ceramic reinforced with an oxide ceramic fiber and which has high toughness and is machinable; as well as to a process for producing such a composite material.
2. Description of the Prior Art
Oxide ceramics have been employed, for their oxidation resistance, as a heat-shielding sheet for high temperatures or as a spacer used in a heat treatment of metal or the like in high-temperature oxidizing atmosphere.
Monolithic oxide ceramics easily give cracking or warpage when subjected to thermal shock and are therefore unusable in the above applications for a long period of time. Moreover, these ceramics are inferior in mechanical impact resistance and, when subjected to mechanical impact, give much chipping and cracking; therefore, they need elaborate handling. For example, alumina ceramics are generally known to be inferior in thermal shock resistance and mechanical impact resistance.
In order to obtain an improved resistance to thermal shock, there are ceramic materials comprising a non-oxide ceramic of high thermal shock resistance and an oxidation-resistant film formed thereon.
Also, there are being investigated materials comprising a carbon material of low oxidation resistance and an oxidation-resistant ceramic layer formed thereon.
The above non-oxide ceramic or carbon material having an oxidation-resistant film or layer formed thereon, however, has defects (e.g. cracks) in the surface film or layer, in some cases. Further, in the surface film or layer, cracks tend to generate owing to the thermal expansion difference between the base material and the surface film or layer. The presence of such defects and cracks accelerates the oxidation of the inner base material, posing a limit to an increase in the life of the base material. Thus, the technique of covering the surface of a base material with a film or a ceramic layer is low in reliability with respect to oxidation resistance.
Meanwhile, there are composite materials of various kinds, obtained by reinforcing an oxide ceramic with ceramic particles or whiskers.
These composite materials obtained by reinforcing an oxide ceramic with ceramic particles or whiskers have shown some improvements in toughness to impact fracture. However, they are not improved in fragility which is the biggest weakness of ceramic materials, and still have a drawback that once they start fracture, it leads to total fracture owing to the fragility.
There have also been developed composite materials obtained by reinforcing an oxide ceramic with an oxide fiber.
These composite materials, unlike the monolithic oxide ceramics, are confirmed to have a large fracture resistance. When in producing such a composite material, the oxide ceramic and the oxide fiber react with each other during sintering, the resulting composite material is very low in strength. In order to avoid such a reaction, a reaction-preventive coating material has been applied to each oxide fiber. Incidentally, the coating material is applied to each fiber uniformly in a thickness of several microns or less {e.g. Advanced Ceramic Materials, Vol. 3, [3], 235-237 (1990); Ceram. Eng. Sci. Proc., 11 [9-10], 1761-1777 (1990)}.
FIG. 2 is a schematic view showing the section of a conventional oxide ceramic fiber/oxide ceramic composite material.
As shown in FIG. 2, in a conventional oxide ceramic fiber/oxide ceramic composite material 200, ceramic fibers 20 are dispersed in an oxide ceramic matrix 22. The surface of each ceramic fiber 20 is covered uniformly with a coating material 24 of several microns or less in thickness. The coating material 24 separates each ceramic fiber 20 from the matrix 22 and prevents their reaction.
In some of the composite materials obtained by reinforcing an oxide ceramic with an oxide fiber, the oxide fiber is coated with a porous oxide (e.g. Al2O3 or ZrO2), BN or carbon {Ceram. Eng. Sci. Proc., 9 [7-8] 705-720 (1988)}, or with monazite under controlled conditions (U.S. DOE Reports DOE-MC-32085-99-Vol. 2).
When an oxide ceramic is reinforced with an oxide fiber bundle consisting of large number of fibers, however, it is difficult to coat the surface of each fiber of the fiber bundle uniformly with the coating material 24. Moreover, since the coating material 24 peels off easily, the fibers after coating must be handled elaborately.
If in producing the above composite material, part of the coating material 24 is detached from each fiber 20, and each fiber 20 and the matrix ceramic 22 come into direct contact, the composite material is deteriorated in strength at the site of direct contact. As a result, the merit of using the fibers for obtaining a composite material is reduced.
Further, elaborate handling of the fibers, prevention of the coating material from peeling, and realization of optimum interfacial state require a very complicated operation, leading to an increase in production cost.
Furthermore, in selecting the raw materials for use in production of composite material, it is necessary to consider the fiber, the coating material and the oxide from the overall standpoints of their chemical and physical properties and judge the mutual compatibility of the raw materials. Therefore, the selection range of the raw materials is restricted and the selection of the raw materials most suitable for an intended composite material is restricted.
The present inventors made a study in order to solve the above-mentioned problems. As a result, the present inventors found out that the above problems could be solved by, in producing an oxide ceramic fiber/oxide ceramic composite material, first producing a primary composite material and then producing a secondary composite material using the primary composite material. This finding has led to the completion of the present invention.
Objects of the present invention are to provide an oxide ceramic fiber/oxide ceramic composite material which is superior in high-temperature oxidation resistance and thermal shock resistance, which is free from fragility (this is a weak point of oxide ceramics) and which is easy to produce; and a process for producing such a composite material.
The above objects are achieved by the present invention which follows.
1. An oxide ceramic fiber/oxide ceramic composite material comprising:
primary composite materials each consisting of (a) an assembly of ceramic fibers composed mainly of a metal oxide and (b) a metal oxide ceramic which includes the ceramic fiber assembly (a) therein, the metal oxide of the ceramic (b) being different from the main component metal oxide of the ceramic fiber assembly (a) and the amount of the metal oxide ceramic (b) being 1 to 85% by weight relative to the weight of the ceramic fiber assembly (a), and a metal oxide ceramic which is a matrix for the primary composite materials and which includes the primary composite materials therein, the metal oxide of the ceramic being the same as or different from the main component metal oxide of the ceramic fiber assembly (a), which composite material is integrally sintered and has a relative density of 20 to 95% as compared with the density of the total metal oxides in the composite material.
2. A composite material according to the above 1, wherein the main component metal oxide of the ceramic fiber assembly (a) is at least one kind of metal oxide selected from alumina, silica, alumina-silica, mullite, titania, YAG and zirconia.
3. A composite material according to the above 1, wherein the metal element of the metal oxide ceramic (b) including the ceramic fiber assembly (a) therein is an element selected from group IV (titanium group), group V (vanadium group) and group VI (chromium group) of the periodic table.
4. A composite material according to the above 1, wherein in any section of the composite material, the fiber axes of different ceramic fiber assemblies are aligned in various directions and the angles formed by the fiber axes are 30xc2x0 or more.
5. A composite material according to the above 1, wherein each ceramic fiber of the ceramic fiber assembly (a) is a ceramic fiber containing 25% by weight or more of Al2O3 and the ceramic fibers are contained in the composite material in an amount of 5% by volume or more.
6. A composite material according to the above 1, wherein the ceramic fiber assembly (a) is a fabric wherein the axes of the fibers are aligned in one direction.
7. A composite material according to the above 1, wherein the ceramic fiber assembly (a) is a fabric wherein the axes of the fibers are aligned two-dimensionally.
8. A composite material according to the above 1, wherein the ceramic fiber assembly (a) is a fabric wherein the axes of the fibers are aligned multi-dimensionally.
9. A composite material according to the above 1, wherein the metal oxide of the ceramic (b) different from the main component metal oxide of the ceramic fiber assembly (a) consists of two kinds or more of metal oxides different in chemical composition.
10. A composite structure which is a sintered material between a ceramic structure and an oxide ceramic fiber/oxide ceramic composite material set forth in the above 1.
11. A process for producing an oxide ceramic fiber/oxide ceramic composite material having a relative density of 20 to 95% by weight as compared with the density of the total metal oxides in the composite material, which process comprises:
allowing a metal or metal oxide to include therein an assembly of ceramic fibers composed mainly of a metal oxide to obtain a primary composite material, the metal or metal oxide being different from the main component metal oxide of the ceramic fiber assembly and being used in an amount of 1 to 85% by weight as compared with the weight of the ceramic fiber assembly,
allowing a metal oxide which is the same as or different from the main component metal oxide of the ceramic fiber assembly, to include therein the primary composite material to obtain a secondary composite material, and
firing the secondary composite material in an oxidizing atmosphere to obtain an integral sintered material.
12. A process for producing a composite material according to the above 11, wherein the main component metal oxide of the ceramic fiber assembly is at least one kind of metal oxide selected from alumina, silica, alumina-silica, mullite, titania, YAG and zirconia.
13. A process for producing a composite material according to the above 11, wherein the metal element of the metal or metal oxide including therein the ceramic fiber assembly is an element selected from group IV (titanium group), group V (vanadium group) and group VI (chromium group) of the periodic table.
14. A process for producing a composite material according to the above 11, wherein each ceramic fiber of the ceramic fiber assembly is a ceramic fiber containing 25% by weight or more of Al2O3 and the ceramic fibers are contained in the composite material in an amount of 5% by volume or more.
15. A process for producing a composite material according to the above 11, wherein the metal oxide different from the main component metal oxide of the ceramic fiber assembly consists of two kinds or more of metal oxides different in chemical composition and the firing in oxidizing atmosphere to obtain an integral sintered material is conducted at temperatures of 1,000xc2x0 C. or more for 30 minutes or more.
The composite material of the present invention has high thermal shock resistance. Further, the present composite material has a feature that even when a small mar generates therein, the mar does not lead to the total fracture of the composite material. Therefore, the present composite material can be subjected to cutting, drilling, etc. The present composite material is flexible when it is a thin material. The present composite material can also be produced in a large size.
The composite material of the present invention has a relative density of 20 to 95% and has voids inside. These voids suppresses the spreading of the cracks generating in the composite material during the thermal expansion or shrinkage, and function like a buffer material. Further, the present composite material is superior in thermal shock resistance in high-temperature oxidizing atmosphere.
The composite material of the present invention has flexibility because no complete bonding is formed between the matrix and the individual fibers. As a result, even when the composite material undergoes a mechanical impact, the composite material is hardly fractured, and it is suited for applications such as electrical insulating material having a curved surface.
Therefore, the composite material of the present invention is suited for insulating materials for semiconductor diffusion furnace, CVD apparatus, high-temperature electric furnace, etc.; heat-resistant spacers used in heat treatment of metals, etc.; high-temperature gas filters; molten metal filters; filter burners; burners; gas turbine engine members such as ceramic burner diffuser and the like; and so forth.
In producing the composite material of the present invention, an oxide hardly reactive with ceramic fibers is allowed to include ceramic fibers therein, to produce a primary composite material, after which a metal oxide is allowed to include the primary composite material therein, to produce a secondary composite material. Thus, unlike in production of conventional composite materials, it is not necessary to coat individual ceramic fibers with the oxide hardly reactive with ceramic fibers, in order to prevent a reaction between the ceramic fibers and the metal oxide. As a result, the present composite material can be produced simply.